A guinea, new from the mint, will re quire 129 grains to be offered to its weight in air; but, on being immersed in water, will require 7+ grains more to re store the equilibrium lost by the immer sion. From this we see, that a quantity of water equal in bulk with the guinea weighs 7+ grains, or 7.25, by which di vide 129, (the weight in air,) and the quotient will be 17.793; shewing that the guinea is as 17.793, to one of water.
But we sometimes have occasion to as certain the precise. weight'of bodies that are lighter than water, say a piece of cork, and which, if unaided, would flat on its surface. In such case, it is necessary to affix a weight (having previously found its exact poise) thereto ; when, by im mersing both, and deducting the amount of the collateral weight, the residue will be left to account of the cork. If you would weigh quicksilver, it must be first balanced in a glass bucket, of which the weight is known, and which has been weighed also by immersion. When the bucket has been brought to equilibrium in the water, pour in the quicksilver; and the additional weight requisite to coun terbalance it will show its exact weight.
Perhaps the following general rules for finding the specific gravity of bodies may prove useful and familiar to every under standing. 1, "When the body is heavier than water." Weigh it both in water, and in the atmosphere, and the differ. ence between the results will show the quantity lost in the former mode ; then as the weight lost in water is to the weight in air, so is the gravity of water to the gravity of the body. 2. "When the body, being specifically lighter, will not sink in water." Render the body heavy enough to sink means of some appendage, as a small piece of lead, Sec.; weigh the body and the appendage, both separately and together, in the air, and in the water ; find out how much each loses in the water, and subtract those losses from the whole weight of each in air. Then, as the last remainder into the weight the light body in air, so is the gravity of water to the gravity of the bo dy. o. "When a fluid is to be weighed." Weigh the fluid in a cup, which is to be deemed an appendage, and treated ac cording to the foregoing rule, observing, that as the whole weight is to the loss of weight, so is the gravity of the solid to the gravity of the fluid.
'We may the respective weights of two known ingredients in a given compound thus take the differen ces of every pair of the three specific gravities; (viz. the specific gravities of the compound, and of 'each ingredient :) multiply each, quantity by the difference of the other two; then, as the greatest product is to the whole weight of,. the compound, so is each of the other two products to each respective weight of the two ingredients.
If a piece of glass, or of metal, be im mersed by suspension in different fluids, it will lose in weight; that is, it will re quire an equipoise, according ' to the weight of the fluids respectively : observ ing, that in the lightest fluid, say alcohol, it will lose least weight. This is the prin ciple on which the hydrometer acts, as will be subsequently shown.
Vessels filled with water weigh more than when empty : to prove this, let a bottle be loaded so is to sink in a pail of water, deep enough for the water to corer its mouth, which should be pre viously closed by a plug, in such man ner as might be easily puShed in ; ap pend the bottle in equilibrio to the hy drostatic balance, and drive in the plug :. the water will follow and destroy the equilibrium.
'Fluids press every away alike, though their general tendency is to gravitation. Thus, if a vessel be made weaker in the side than at the bottom, and be so laden or oppressed by the weight of water, as to burst the vessel, the weakest part, wherever situated, will become the out let ; but, so soon as liberated, the fluid will inl;ariably descend, unless acted upon by a syphon, as shown in treating of hy draulics. The pressure upwards is, how ever, merely in conformity with circum stances attendant upon general pressure, and proves the tendency of fluids to find their own level. Thus, if you take a glass tube of moderate diameter, open at both ends, and stop one closely with your fin ger, when you immerse the other end in any fluid, it will enter but little within the vacancy ; because the columns of air within the tube repress it. But when the finger is withdrawn, the water will as cend within the tube, to the level of the body in which it is immersed.